1. Field of the Invention
The present invention relates to a technique for reducing wavelike distortions or curling of sheets by applying moisture or an aqueous solution to the sheets in an image forming apparatus utilizing an electrophotographic system.
2. Description of the Related Art
Conventionally, an image forming apparatus utilizing an electrophotographic system develops a latent image formed on a photosensitive drum serving as an image bearing member, to visualize an image. The image forming apparatus transfers the visualized image (a toner image) onto a sheet, using an electrostatic force. The transferred image is then thermally fixed, so that the image is formed on the sheet.
In image forming apparatuses such as electrophotographic apparatuses and electrostatic recording apparatuses, a toner image is formed on a sheet, and heat and pressure are applied onto the toner image, so as to fix the toner image. In this manner, an image is formed. As the fixing device which performs such operations, there has been a roller fixing system which presses a pressure roller against a heater-containing fixing roller to form a fixing nip portion, and performs image fixing.
In the fixing devices provided in image forming apparatuses such as copying machines utilizing electrophotographic systems, thermal-roller fixing devices have been often used to fix a toner image onto a sheet.
A fixing device utilizing thermal roller system guides a sheet into a pressing nip portion (a fixing nip portion) that is formed by a fixing roller heated by a built-in heat source such as a halogen heater and maintained at a predetermined temperature, and a pressure roller that is pressed against the fixing roller and has elasticity. The fixing device nips and conveys the sheet. An unfixed toner image on the surface of the sheet is then thermally fixed. Since heat and pressure are applied onto the toner and the sheet in this process, the moisture in the sheet evaporates while the sheet is being pressed.
Due to variation in the amount of moisture in the sheet and the stress applied to the sheet, wavelike distortions and curling appear in the sheet. When a paper sheet is observed in terms of fiber, paper is formed by short fibers that are tangled up, and moisture exists in or between the fibers. Therefore, the fibers and the moisture form hydrogen bonding.
When heat and pressure are applied onto a paper sheet in an image fixing process, the fibers deviate from one another due to the pressure. As the paper sheet is heated, and moisture evaporates, hydrogen bonding occurs between the fibers, resulting in deformation of the sheet. If the paper is left, the paper absorbs moisture from the surroundings, and the hydrogen bonding between the fibers is again broken. However, moisture does not reach some of the fibers, and the deformation of the sheet remains. Deformation may be caused by the difference in expansion and contraction rates between the front and back surfaces of a paper sheet, or may be caused by the difference in expansion and contraction rates between the center portion and each end portion of a paper sheet. Such deformation leads to wavelike distortions or curling of the sheet.
Japanese Patent Laid-Open No. 11-167317 discusses a solution to the above problem. The technique according to Japanese Patent Laid-Open No. 11-167317 concerns a device and system used in electrostatic copying machines. This device and system prevent formation of curling and wavelike distortions at the edges of paper sheets for copiers due to evaporation of moisture from the paper sheets in electrostatic copying or printing fixing steps. This device includes a water-jet unit that applies a controlled amount of moisture to one of or both of the surfaces of a copy sheet. The water-jet unit is positioned on both surface sides of the copy sheet, and has a reservoir for holding liquid. This device also includes a pair of pressure rollers each having a cylindrical external surface. The pair of pressure rollers are positioned to face each other along the shafts thereof, so that a nip portion can be secured between the cylindrical external surfaces. This device further includes a control device that controls the amount of moisture to be applied from the water-jet unit to a selected portion of each sheet passing through this device, before the sheet enters the nip portion formed between the cylindrical external surfaces. The amount of moisture in each sheet decreases after the sheet passes through the fixing device, and curling and wavelike distortions appear in the sheet. To counter this phenomenon, moisture is applied to the sheet after the fixing, and the lost moisture is compensated for. In this manner, curling and wavelike distortions are made smaller.
By applying moisture to paper, the moisture having evaporated due to the heat and pressure in the image fixing process is compensated for, and wavelike distortions and curling are corrected. As the amount of moisture in paper becomes larger, the Young's modulus of the paper becomes lower. The stiffness of the paper then becomes lower, and the wavelike distortions become smaller. It is also believed that, when moisture enters spaces between fibers, the bonding between the fibers having formed the hydrogen bonding is again broken by the fixing device, and accordingly, wavelike distortions become smaller.
By the technique discussed in Japanese Patent Laid-Open No. 11-167317, curling or wavelike distortions of sheets are temporarily reduced by applying moisture to the paper sheets. However, as the applied moisture evaporates and the sheets become dry, the curling or wavelike distortions of the sheets again increase.